There really is no difference for a strictly gaming machine.
Both the P67 & Z68 can handle the new Ivy Bridge processors when they come out and they can both do PCIe3.0. Neither can do native USB3.0 support. The only benefits of Z68 over P67 is integrated video (quick sync) and the Intel Smart Response Technology (SSD caching).
Get whatever one fits your budget better unless you want quick sync support. Personally I wouldn't get anything above the ASRock Extreme4 Gen3 (Z or P since it comes in both flavors), so I'd spend about $200 max on a motherboard. Most motherboards above that level come with features you'll probably never use, like an extra Gb Ethernet port, 3 extra PCIe slots, 4 extra SATAIII ports (only useful if you have 6 total SSDs), and so on. You'd be better off putting the money into other parts of the PC. The only exception would be if you knew you'd actually need a specific feature like 3-way SLI/Crossfire.